Calorie restriction (CR) is known to extend the lifespan of rodents and several ?lower? species, and to improve surrogate markers for longevity and mortality in rhesus monkeys. There is, however, a paucity of reliable information on whether this is the case in humans as well. In response to a specific RFA from the NIA for descriptive experiments to address this issue, we propose a controlled, single-blinded study of several adaptive responses pertinent to this question in overweight volunteers imposed an initial 25% negative energy balance for two years by CR alone or by CR with additional physical activity. These responses include changes in energy metabolism, surrogate markers of oxidative stress and risks for cardiovascular disease and type-2 diabetes mellitus, neuroendocrine functions and autonomic nervous system activity, expression of candidate genes involved in energy metabolism, oxidative stress and longevity, and quality of life and physical fitness. In addition to descriptive information, we will test the hypotheses that CR with weight loss and maintenance of energy balance at a new body weight: 1. Lowers absolute and relative rates of energy expenditure and body temperature and decreases evidence of tissue oxidative stress. 2. Improves surrogate markers for cardiovascular disease and type-2 diabetes mellitus. 3. Affects the diurnal secretion of leptin leading to adaptations in the neuroendocine and autonomic nervous system enhance soma maintenance. 4. Alters the expression of genes involved in energy/protein metabolism, the production and clearance of reactive species, and ?longevity? genes in a fashion consistent with animal models of CR. 5. Improves the quality of life and metabolic fitness. A multidisciplinary team, including physiologists, behavioral psychologists, endocrinologists, molecular biologists and geneticists will conduct this study.